1. Field of the Invention
This invention relates to a thermal printer for recording an image on a recording sheet. Thermal printers include, for example, facsimile apparatuses, word processors or copying apparatuses. Recording sheets include, for example, thermosensitive recording sheets or plain paper recording sheets.
2. Related Background Art
A description will hereinafter be provided of a facsimile apparatus taken as an example of the thermal printer.
For example, in the ordinary facsimile apparatus, recording is accomplished by a heat reaction in thermosensitive recording paper. In such a construction, various alkali ions such as Na.sup.+ are contained in the thermosensitive recording paper to prevent generation of static electricity. However, when such thermosensitive recording paper absorbs moisture the alkali ions in the thermosensitive recording paper dissolve into water, whereby an electrolyte solution is made. This electrolyte solution enters into the thermal head through pin-holes in the protective film of the thermal head. Here, supposing a condition in which a power source is supplied to the recording unit of the thermal head and the recording (printing) cycle is not going on, there is brought about a state in which a voltage V is normally applied with the electrode of the thermal head as the positive pole and with the platen roller as the negative pole. Thus, an ion current flows from the electrode of the thermal head to the platen roller through the heat generating resistance members and further through the pin-holes in the protective film and the electrolyte solution in the thermosensitive recording paper. As a result, the electrode, etc. are electrolyzed, and if this current continues to flow for a long time, the electrode will be destroyed at last and recording will become impossible.
Heretofore, to prevent such phenomenon, control has been effected so that no voltage is applied to the recording unit of the thermal head, for example, during facsimile transmission (an example of the non-recording cycle). Specifically, a relay circuit or the like has been provided for control so that the supply of power to the recording unit of the thermal head is cut off or the voltage of the power source circuit itself is not generated. Therefore, it has been necessary to add a relay of great capacity to the load side of the power source unit or add an ON/OFF control circuit to the power source unit itself.